Sulfonation of pulp produced by alkali pulping process

ABSTRACT

This invention relates to the delignification of lignocellulosic pulp. More particularly, the invention relates to sulfonation and bleaching of the alkali pulping produced by Kraft pulp processes or produced through combination of alkali pulping processes and extended delignification stages.

FIELD OF THE INVENTION

This invention relates to the delignification of lignocellulosic pulp.More particularly, the invention relates to sulfonation and bleaching ofthe alkali pulping produced by Kraft pulp processes or produced throughcombination of alkali pulping processes and extended delignificationstages.

BACKGROUND OF THE INVENTION

Wood is composed of two main parts, namely, a fibrous carbohydrate orcellulosic portion and a non-fibrous portion comprising a complexchemical, commonly referred to as lignin.

For use in papermaking processes wood must first be reduced to pulp,which can be defined as wood fibers capable of being slurried orsuspended and then deposited as a screen to form a sheet. The methodsemployed to accomplish this pulping usually involve either physical orchemical treatment of the wood, or perhaps some combination of the twoprocesses, to alter its chemical form and to give desired paperproperties.

In chemical pulping, the wood chips are digested with chemical solutionsto solubilize a portion of the lignin and the effect its removal of thelignin. The more usual of these digestive procedures are the sulfite,sulfate or Kraft, modified sulfite and soda processes.

Common industrial practices for the production of fully bleachedchemical pulp grades entail two major unit operations namely 1) Cookingto transform wood chips into pulp and to dissolve associated lignincontent, from more than 20% lignin in wood chips to less than 5% ligninin pulp and 2) Bleaching to substantially remove residual lignin in pulpand to increase the pulp brightness.

It is well known that from identical wood species and cooking to similarresidual lignin content, pulps produced with alkali pulping processessuch as Kraft are stronger but darker in color and more difficult tobleach than pulps produced under acidic conditions such as those usedfor acid sulfite pulping process.

Caustic, carbonate, sulfide, sulfite, chemical additives or any of theircombination are known to be used for alkali pulping processes. Pulpswith lower lignin content and higher brightness can also be producedthrough combination of alkali pulping and an extended delignificationstage using bleaching chemicals such as hydrogen peroxide, oxygen orozone.

Use of solutions of SO₂, MHSO₃ and M₂SO₃ on wood chips is well known insulfite pulping industry (Pulp and Paper Manufacture Vol. 4. SulfiteScience & Technology, book published by Canadian Pulp and PaperAssociation). Sulfonation or use of solution of SO₂ or combination ofSO₃ and SO₂ on pulps has also been investigated as means to increaseassociated paper making properties (K. Kringstad, J. Olausson, SvenskPapperstidning n.13, P480-485, 1974) (U.S. Pat. No. 5,522,967). Use ofSO₂ as an alternative source of acid for pulp washing and trace metalremoval has also been proposed in U.S. Pat. No. 3,725,194, and U.S. Pat.No. 4,560,437. However, none of these cited references disclose or teachSO₂ as a treatment to remove lignin or to transform the treated lignin(i.e., modified lignin) in a manner that would be receptive to chemicalreaction or substitution with bleaching chemical during the bleachingsequence.

SUMMARY OF THE INVENTION

This invention relates to sulfonation and bleaching of pulps produced byalkali pulping processes or produced through combination of alkalipulping processes and extended delignification stages.

Accordingly, one aspect of the present invention relates to a method ofmaking pulp comprising sulfonating an alkali pulp at a pH less than 7prior to, during, or after at least one bleaching sequence. Thebleaching sequence occurs after extended delignification stage. Thesulfonating of the alkali pulp is generally followed by a washing stage.

Another aspect of the present invention relates to a sulfonated and/orbleached alkali pulp comprising a composition of cellulose,hemicelluloses wherein the amount of cellulose contained in the pulp isfrom about 55% to about 75% by dry weight of the pulp and the amounthemicellulose contained in the pulp is from about 25% to about 45% bydry weight of the pulp that also containing less than about 5% lignin.

One further aspect of the present invention relates to a paper orpaperboard comprising a sulfonated and bleached alkali pulp wherein theamount of cellulose contained in the pulp is from about 55% to about 75%by dry weight of the pulp and the amount hemicellulose contained in thepulp is from about 25% to about 45% by dry weight of the pulp that alsocontaining less than about 5% lignin.

DETAILED DESCRIPTION OF THE INVENTION

It is advantageous to define several terms before describing theinvention. It should be appreciated that the following definitions areused throughout this application.

Definitions

Where the definition of terms departs from the commonly used meaning ofthe term, applicant intends to utilize the definitions provides below,unless specifically indicated.

For the purpose of the present invention, the term “Kraft Process”refers to alkaline chemical pulping process using sodium hydroxide(NaOH) and sodium sulfide (Na₂S) as the active cooking chemicals. Theprocess is noted for producing the strongest pulps. The Kraft process isalso defined as sulfate chemical pulping process. In addition, anyequipment used as well as any intermediate or final products derivedfrom the process.

For the purpose of the present invention, the term “Kraft Pulp” refersto wood pulp produced by the Kraft sulfate chemical process usingcooking liquor. This liquor is made up primarily of sodium hydroxide(NaOH) and sodium sulfide (Na₂S), using basically softwood species ofpulpwood, but could be hardwood. Kraft pulp is also known as sulfatepulp.

For the purpose of the present invention, the term “Alkali” refers toSodium-based chemicals such as those found in sulfate pulping liquors.

For the purpose of the present invention, the term “Alkaline pulping”refers to the process of cooking wood chips with alkaline-basedchemicals. Examples of such include, but not limited to, Kraft or sodachemical pulping processes. The term may be applied to other high pH(e.g., 8 to 14) chemical pulping processes such as soda-oxygen oralkaline sulfite or sulfate.

For the purpose of the present invention, the term “Bisulfate process”refers to Sulfite pulping process in which the cooking liquor is in thepH range from 2 to 6 and contains a predominance of bisulfite ion (HSO₃⁻) with little or no true free SO₂. Bases used are magnesium, sodium andammonium.

For the purpose of the present invention, the term “Acid Pulping” refersto a general term usually denotes the sulfite process. However, the termcould apply to any low pH (e.g., 2 to 4) chemical pulping process suchas (the experimental) nitric acid pulping.

For the purpose of the present invention, the term “Acid sulfiteProcess” refers to sulfite pulping process in which the cooking acidcontains a high percentage of free sulfur dioxide (SO₂) relative tocombined SO₂ and therefore has a very low pH (e.g., 1 to 2). The basemay be calcium, sodium, magnesium or ammonium.

For the purpose of the present invention, the term “Sulfate Process”refers to an alkaline pulp manufacturing process in which the activecomponents of the liquor used in cooking chips in a pressurized vesselare primarily sodium sulfide (Na₂S) and sodium hydroxide (NaOH) withsodium sulfate (Na₂SO4) and Lime (CaO) being used to replenish thesechemical in recovery operations. Sometimes referred to as the Kraftprocess.

For the purpose of the present invention, the term “Sulfate Pulp” refersto fibrous material used in pulp, paper, and paperboard manufacture,produced by chemically reducing wood chips into their component parts bycooking in a vessel under pressure using an alkaline cooking liquor.This liquor consists primarily of sodium sulfide (Na₂S) and sodiumhydroxide (NaOH). The term sometimes referred to as the Kraft pulp.

For the purpose of the present invention, the term “Sulfonation” refersto a chemical reaction in which lignin is reacted with sulphonationchemical to form lignin sulfonates. Suitable sulfonating agents for thepurposes of this invention includes, but not limited to, solution ofsulfur dioxide (SO₂) or sulfur trioxide (SO₃) or sulfurous acid (H₂SO₃),sodium metabisulfite (Na₂S₂O₅) hi-sulfite (MHSO₃) or sulfite (M₂SO₃)where M can be Na, Ca, H, K, Mg or their combination.

For the purpose of the present invention, the term “Delignification”refers to removal of all or part of the lignin from wood or plantmaterial by chemical treatment. Chemical pulping and the initial stagesof bleaching are examples of delignification. Examples of the initialstages of bleaching are, for example, C (Chlorination), D₀ (initialdelignification), D₁ (first brightening), and D₂ (second brightening).

For the purpose of the present invention, the term “sulfur trioxide(SO₃)” refers to toxic gas formed by further oxidation of sulfur dioxide(under certain condition) which forms sulfuric acid (H₂SO₄) whendissolved in water. Sulfur trioxide is an undesirable by product whenburning sulfur for sulfite cooking liquor preparation. In sulfuric acidplants, a heated catalyst is used to promote oxidation to the trioxideform. Any delignification following pulping would be considered extendeddelignification including the first stages of a delignification and/orbleaching sequence.

For the purpose of the present invention, the term “ExtendedDelignification” refers to any modification of the pulping process whichallows for greater delignification of the pulp without adverse effectson pulp strength, i.e., produces a lower Kappa number pulp. Examples ofcommercial extended delignification processes are Modified ContinuousCooking and Rapid Displacement Heating.

For the purpose of the present invention, the term “DelignificationIndex” refers to measurement of the degree of delignification. Acommonly used delignification index is the Kappa number test.

For the purpose of the present invention, the term “Lignosulfonates”refers to compounds formed during sulfite cooking by the reaction ofsulfurous acid or bisulfite ion with the lignin in the wood. Thesecompounds are soluble in the cooking liquor.

For the purpose of the present invention, the term “Bleaching” refers towhitening process carried out on pulps by selective chemical removal ofresidual lignin and other colored materials, and with minimaldegradation of the cellulosic constituents. With respect to secondaryfibers, bleaching can also have a dye removal function.

For the purpose of the present invention, the term “Bleaching Chemical”refers to a variety of chemical used in the bleaching of wood pulp suchas chlorine (Cl₂), sodium hypochlorite (NaOCl), calcium hypochlorite[Ca(OCl)₂], chlorine dioxide (ClO₂), peroxide (H₂O₂), sodium chlorite(NaClO₂), Oxygen (O₂), Ozone (O₃) and others.

For the purpose of the present invention, the term “Bleaching Sequence”refers to series of stages, each with specific objectives (e.g.,delignification, solubilization, destruction of chromophoric groups),that contribute to an overall whitening effect. Typically the pulp iswashed between stages.

For the purpose of the present invention, the term “Kappa Number” refersto modified permanganate test value on pulp which has been corrected to50% consumption of chemical. Kappa number has the advantage of a linearrelationship with lignin content over a wide range. For pulp samplesunder 70% yield, the percent Klason lignin is approximately equal to theKappa number times a factor of 0.15.

For the purpose of the present invention, the term “Buffer” refers tochemical solution that resists change in pH when acids or alkalis areadded. It could include, but not limited to, NaOH, and H₂SO₄.

For the purpose of the present invention, the term “Buffering Action”refers to ability to neutralize acids and bases as they are formedduring a chemical reaction and thus resist a change in pH.

For the purpose of the present invention, the term “OxygenDelignification (O-Stage)” refers to treatment of pulp in alkalinemedium with oxygen to degrade and solubilize lignin, typically employedas the first stage of a bleaching sequence or as a bleaching“pre-stage”. The process is generally carried out at “mediumconsistency”. Oxygen is added as a gas and magnesium salts are usuallyemployed as an additive to “protect” the cellulose from degradation.

For the purpose of the present invention, the term “Ozonation Stage(Z-Stage)” refers to delignification or bleaching treatment of pulp withozone, carried out under acidic conditions. The ozonation stage istypically used at the beginning of a bleaching sequence where itsdelignification efficiency is best and where degradation of cellulose isretarded by the high amount of lignin still present in the pulp.

For the purpose of the present invention, the term “Chlorine-BasedDelignification Stage, aka, D-Stage” refers to initial delignifyingstage (D₀) and/or brightening stages (D₁ and D₂) in a bleaching sequencein the process of a chorine-based bleaching chemical, used to producehigh-brightness pulp. Traditionally, the highest and most stablebrightness (especially for the softwood Kraft pulps) is achieved when atleast two chlorine dioxide brightening stages are used with an alkalineextraction in between.

For the purpose of the present invention, the term “Alkaline ExtractionStage-(E-Stage)” refers to essential stage in any multistage bleachingsequence; it solubilizes the dark-colored chlorinated and/or oxidizedlignin compounds formed in the initial acid delignification stage (e.g.,chlorination or chlorine dioxide) and in later stages. When used priorto the final bleaching stage, an E-stage also serves to “activate” thepulp for more effective brightening.

For the purpose of the present invention, the term “Peroxide ExtractionStage-(Ep-Stage)” refers to alkaline extraction stage supplemented witha peroxide.

For the purpose of the present invention, the term “Oxidative ExtractionStage-(Eo-Stage)” refers to alkaline extraction stage supplemented withan oxidizing agent, most commonly oxygen. Peroxide or hypochlorite mayalso be used as supplemental chemicals to provide a brightening effectand/or to reduce effluent color.

For the purpose of the present invention, the term “Oxidative peroxideExtraction Stage-(Eop-Stage)” refers to alkaline extraction stagesupplemented with peroxide and an oxidizing agent.

For the purpose of the present invention, the term “Brightening” refersto 1) any chemical treatment to pulp that increases its brightness. 2)Chemical modification of colored elements in high-yield pulps to renderthem colorless without removing them, thus retaining the yield advantageof these pulps.

For the purpose of the present invention, the term “Peroxide” may be anyperoxide, but specifically may refers to sodium peroxide (Na₂O₂) orhydrogen peroxide (H₂O₂) which are used to make up bleach liquor forbleaching mechanical-type pulp. Peroxides are used in the bleaching ofboth high-yield and chemical pulps. When used under relatively mildconditions (35 C to 55 C), peroxide is an effective lignin-preservingbleaching agent, improving the brightness of groundwood and other highlylignified pulps without significant yield loss.

For the purpose of the present invention, the term “Consistency” refersto mass or weight percentage of oven dry fiber in a pulp solution, e.g.,pulp and water, or stock (pulp and additives) and water. It is expressedas a percentage of this material in the solution, in terms of bone dry(BD), oven dry (OD), or air dry (AD) weight. Consistency is oftendescribed qualitatively as low, medium, or high without reference to astandard nomenclature. The following ranges are given as a generalguide: very low consistency (0-1%), low consistency (1-8%), mediumconsistency (8-16%), and high consistency (16-40%).

For the purpose of the present invention, the term “retention time”refers to contact period of pulp with a bleaching chemical; usuallymeasured from the point of chemical addition to the point where residualchemical is washed out or displaced by another chemical.

It has now been discovered that unbleached or partially bleached alkalipulp including, but not limited to, alkali pulp can be treated with asolution containing a sulfonating agent to form sulfonated alkali pulp.When sulfonating unbleached Kraft pulp, it is the lignin portion of thepulp that is sulfonated rather than sulfonation of the celluloseportion. The sulfonating agents may be any agent that, when added to thealkali pulp may sulfonate the lignin. Sulfonation of lignin serves tosoften the lignin and/or make it soluble under suitable conditions inthe form of sulfonated lignin or a ligno-sulfonate. The sulfonatedlignin is extracted easier from the alkali pulp and much receptive toreact with bleaching chemicals and therefore the amount of bleachingchemicals used in the bleaching process is significantly reduced. Itgoes without saying that optionally buffering agent can be used.

The D* stage is conducted in an acidic or almost neutral condition andthe process of the present invention provides one or more advantagesover prior bleaching processes known in the art. For example, some ofthe advantages of the embodiments of the process of this inventioninclude 1) significant reduction of bleaching chemicals such as, ClO₂,H₂O₂, O₂, O₃, and Z or any combination of the foregoing, 2) reducing thebleaching cost, 3) relatively low capital requirement, 4) high pulpbrightness and brightness stability, 5) higher pulp viscosity, 6)improved bleached plant performance and stability, 7) reduction infiltrate Adsorbable Organic Halides (AOX) or 8) a combination of two ormore of the aforementioned advantages. Some embodiments of thisinvention may exhibit one of the aforementioned advantages while otherpreferred embodiments may exhibit two or more of the foregoingadvantages in any combination.

The present invention relates to a novel sulfonation step (D*) thatoccur after alkaline pulping and anytime before, during or afterextended delignification or bleaching sequences. The alkaline pulp ofthe present invention is preferably Kraft pulp.

One chemical reaction taking place in carrying out the present inventionwith D* stage is as follows:

With regards to the sulfonation reaction, there are varieties of ways inwhich the lignin chain unit can be sulfonated. The sulfonating agentsmay be any agent that, when added to the Kraft pulp may sulfonate thelignin according to the above chemical reaction. The sulfonation oflignin results in more efficient bleaching process of the alkaline pulp,including Kraft pulp. As an example, treatment of the alkaline pulp withsulfonate before or during an extended delignification and/or bleachingsequence exhibits a similar ISO brightness at much reduced bleachingchemical when compared to the same alkaline pulp without going throughthe use of sulfonation step disclosed in the present invention.

The temperature of the sulfonation reaction can be from about 70° C. toabout 140° C., more preferably from about 80° C. to about 120° C., andmost preferably from about 90° C. to about 100° C. At temperatures below70° C., the reaction proceeds too slowly to be practical. Attemperatures greater than 130° C., the lignin's color turns too darkwhich has adverse effect on the downstream bleaching process.

The pH of the sulfonation reaction can be from about 1 to about lessthan 7, more preferably from about 2 to about 5, and most preferablyfrom about 2 to about 4.

The consistency (CSC) of the pulp in the D* stage may vary widely andany consistency that provides the desired increase in pulp brightnessmay be used. The pulp may be bleached under low consistency conditions(i.e. from about 1% to about 5% based on the total weight of the mixtureof pulp and bleaching chemicals), medium consistency conditions (i.e.from about 6% to about 15% based on the total weight of the mixture ofpulp and bleaching chemicals) or high consistency conditions (i.e. fromabout 16% to about 30% based on the total weight of the mixture of pulpand bleaching chemicals). The consistency is preferably from about 10%to about 14%.

The retention times in the D* stage is defined as the contact timebetween SO₂ and the lignin in the pulp. The retention time in the D*stage will vary widely. Usually, retention times will be at least about30 minutes. Retention times are preferably from about 30 min. to about300 min., and are more preferably from about 60 minutes to about 240min., and are most preferably from about 60 min. to about 120 min.

With regard to the sulfonation reaction, suitable sulfonation reagentsinclude, without limitation, alkali bisulfite, such as sodium bisulfite,and a combination of sodium hydroxide and sulfur dioxide. A preferredreagent is sodium bisulfite (NaHSO₃) or alternatively, sulfur dioxide(SO₂), or sulfur trioxide (SO₃) or sulfurous acid (H₂SO₃), bi-sulfite(MHSO₃) or sulfite (M₂SO₃) where M can be Na, Ca, H, K, Mg or theircombinations. The concentration of sodium bisulfite is not criticalprovided there is an excess over the stoichiometric amount required toproceed with the reaction.

A preferred method of making sulfonated alkaline pulp is to sulfonatethe alkaline pulp such as Kraft pulp with at least about 0.5% sulfurdioxide at a consistency from about 10% to 20% for over two hours atabout 90° C. or above. The alkaline pulp thus produced is thenpreferably washed with the water to remove residual chemicals such assulfate and the dissolved lignin. The sulfonated alkaline pulp is thentransferred to one of the bleaching stages such as D₀, Eop, D₁ and soon.

In one embodiment of this invention, the D* stage can be with or withoutan optional washing stage at any stage in the bleaching process providedthat D* stage pulp is an alkaline pulp. In general, the D* stage mayoccur after alkaline pulping process and at prior to any delignificationor bleaching stage such as those utilized within the extendeddelignification stage. Thereafter, the sulfonated alkaline pulp can bewashed immediately following the D* stage or at any subsequent pointprior to or after a subsequent D₀ delignification stage or for example,prior to the Eop or Ep stage. The D* stage can also occur after earlierdelignification stages as for example an oxygen delignification stageand/or a D₀ delignification stage followed by a subsequent interstagewashing prior to the Eop or Ep stage. The D* stage with interstagewashing can be immediately preceding the Pop or Ep stage, or can beseparated from these stages by one or more other stages.

In another embodiment, after D* stage, the alkaline pulp is preferablywashed in the process prior to D₀ stage. Pulp washing after the D* stageremoves the sulfur dioxide residuals and some residual metals to protectperoxide used in the subsequent Ep or Eop stage. In the preferredembodiment of the invention all or substantially all of the sulfurdioxide residuals are removed by washing after the D* stage.

After the washing stage, the alkaline pulp may be subjected to D₀delignification stage. The delignification of unbleached pulp isachieved through a sequence of further steps including delignificationsteps, bleaching steps, and/or extraction steps. Conventional processesand apparatus can be used in the D₀ delignification stage. See forexample “Pulp Bleaching Principles and Practice of Pulp Bleaching”Carlton W. Dence and Douglas W. Reeve, TAPPI Press, 1996 and referencescited therein.'

Alternatively, after the washing stage, the alkaline pulp may besubjected to Oxygen delignification stage. Oxygen and steam may be addeddirectly to the reactor. The sulfonated alkaline pulping fall throughthe reactor to be diluted and discharged at the bottom. The pulp thenpasses through several stages of washing before proceeding to thechlorination or chlorine dioxide stage in a multi-stage bleachingprocess where chlorine dioxide solution is mixed with pulp and allowsreacting, and then washed as one of the operations making up a completepulp bleaching system. The organic material dissolved in delignificationis ultimately burned in the recovery boiler. Conventional processes andapparatus can be used in the O delignification stage. See for example“Pulp and Paper Manufacture—Vol. 5, Alkaline Pulping” Technical Editors:T. M. Grace and E. W. Malcolm and references cited therein.

One of the advantages of the D* stage, is the reduction of bleachingchemicals during extended delignification or bleaching sequences ascompared to the same or substantially the same extended delignificationor bleaching which do not include the D* stage. For example, thereduction in the amount of bleaching chemical such as chlorine dioxide(ClO₂) is preferably at least about 20%, more preferably at least about30%, and most preferably up to 40% less than the amount of ClO₂.Moreover, caustics and peroxides are reduced as well.

The Inventor has discovered that in the D₀ stage, the brightness of pulpis higher or at least the same than those treatments without D* stage,which indicates the positive impact of D* treatment on the bleachingchemical efficiency and selectivity in the D₀ stage. For example, thebrightness is at least about 1%, preferably at least about 2%, morepreferably from about 3% to about 5% and most preferably from about 6%to about 7% greater than the brightness of the pulp made by the same orsubstantially the same bleaching processes which do not include the D*stage.

In one embodiment of this invention, a delignification is used after D*stage and followed by an extraction stages such as E or Eop.Conventional process parameters employed in these oxidative extractionstage can be described in, for example “Pulp Bleaching Principles andPractice of Pulp Bleaching” Carlton W. Dence and Douglas W. Reeve, TAPPIPress, 1996 and references cited therein. Accordingly, they will not bedescribed in greater detail.

In addition to the D* stage followed by an optional washing stage priorto the D₀ stage or (i.e., D*D₀). Other bleaching sequences such as D₀ D*or D*Eop or D*Ep or D₀D*Eop or D₀ D*Ep are preferred embodiment of theinvention. Bleaching process of this invention may include otherdelignification or bleaching stages as for example bleaching with Cl₂,peroxy acids, chlorine dioxide, ozone and the like, and extractionstages such as extraction with oxygen, ozone, borohydride, chlorinedioxide and the like. Illustrative of such full extended delignificationor bleaching processes as the present invention include are D* (D/C)Eop,(D₀D*)EopD, D*D₀EopD, (D*D₀)EopED, (D*D₀)EDEpEopD, D*Z(E)D, D*Z(Eop)D,D*D₀EpZ(Eop) , D*D₀EpZD(ZD), D*D₀ (Eop)D(ZD), D*D₀(Eop)PP, D*D₀(Eop)DZ,D*D₀EopD₁, OD*D₀EopD₁, D*D₀EopD₁D₂, OD*D₀EopD₁D₂, D*D₀EopD₁EpD₂,OD*D₀EopD₁EpD₂, D*D₀Eop1D₁P and the like in which D is as describedabove and Z is ozone, E is extraction in the presence of base, O isoxygen, P is peroxide, D/C is a mixture of chlorine dioxide andelemental chlorine and two or more symbols in parenthesis indicate anabsence of an intermediate washing stage. The Eop, Ep, and D₀ aredefined herein above. The processes and apparatus used in the D, Z, E,O, P, and D/C are conventional and there are well known in art. See forexample, “Pulp Bleaching Principles and Practice of Pulp Bleaching”Carlton W. Dence and Douglas W. Reeve, TAPPI Press, 1996 and referencescited therein.

The sulfonated alkali pulp of the present invention after the D* stage,whether hardwood or softwood pulp, comprises any amounts of cellulose,hemicelluloses, and lignin. These pulps may have any amounts ofcellulose and hemicelluloses including, but not limited to, from 55-75wt % cellulose and from 25-45 wt % hemicelluloses. These pulps may alsoinclude any amounts of lignin including, but not limited to, less thanabout 1, 2, 3, 4, and 5 wt % lignin. After the D* stage, these pulp maycontain any amount of sulfonated lignin including, but not limited to,from about 1 wt % to about 100 wt % sulfonated lignin based upon totalamount of lignin present in the pulp. Examples include 1, 2, 5, 10, 15,20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 95, and about 100 wt% sulfonated lignin based upon the total lignin present in the pulp,including any and all sub ranges therein. Furthermore, in one example,softwood (SW) species pulp comprises about 45% cellulose, about 25%hemicelluloses, about 25% lignin, and about 2-5% extractives. In anotherexample, hardwood (HW) species pulp comprises about 45% cellulose, about31% hemicelluloses, 21% lignin, and about 2-5% extractives. The inventorof the present invention has discovered that by implementing the D*stage in a bleaching sequence, the lignin for the HW pulp issignificantly impacted and preferably reduced. For example, Kappa numberfor the HW pulp can be reduced as much as 40 to 50%.

The Kappa Number of the sulfonated HW Kraft pulp can vary widely. TheKappa number is preferable form about 4 to about 10 and is morepreferably from about 6 to about 8.

The plant source of pulp for use in this invention is not critical andmay be any fibrous plant which can be subjected to chemical pulpbleaching. Examples of such fibrous plants are trees, including hardwoodfibrous trees such as aspen, eucalyptus, maple, birch, walnut, acaciaand softwood fibrous trees such as spruce, pine, cedar, includingmixtures thereof. In certain embodiments, at least a portion of the pulpfibers may be provided from non-woody herbaceous plants including, butnot limited to, kenaf, hemp, jute, flax, sisal, or abaca although legalrestrictions and other considerations may make the utilization of hempand other fiber sources impractical or impossible. The source of pulpfor use in the practice of this invention is preferably hardwood andsoftwood fibrous trees, more preferably Eucalyptus, Spruce and Aspen andis most preferably Aspen and Spruce.

The pulp used in the process of this invention can be obtained bysubjecting the fibrous plant to alkaline pulping process. Following thewood digestion process, pulp is separated from the spent pulping liquor.The spent pulping liquor is then recovered and regenerated forrecycling. The pulp is then bleached and purified in a bleach plantoperation. Such operation includes the D* stage of the presentinvention.

The pulp of this invention can also be used in the manufacture of paperand packaging products such as printing, writing, publication and coverpapers and paperboard products. Illustrative of these products andprocesses for their manufacture are those described in U.S. Pat. Nos.5,902,454 and 6,464,832 and See for example, “Handbook for Pulp andPaper Technologists” G. A. Smook, 1982.

In another example, the alkaline pulp of this invention may be convertedinto fluff pulp as well as absorbent articles such as diapers, tissues,towels, personal hygiene products, and automotive filters usingconventional processes. Such products and their methods of manufactureare known to those of skill in the art and will not be described indetail. See for example, U.S. Pat. Nos. 6,063,982 and 5,766,159 andreferences described therein. The alkaline pulp of this invention can beused to make saturating alkaline paper. Saturating Kraft paper is apaper sheet made from unbleached Kraft pulp (mixture of mostly hardwoodand some softwood such as southern pine) that is used as substrate forimpregnation and curing with resin polymers. Saturating alkaline paperis used as home and office building materials, such as kitchen countertops.

The present invention is described in more detail by referring to thefollowing examples and comparative examples which are intended to morepractically illustrate the invention and not to be a limitation thereon.

EXAMPLE 1

An unbleached hardwood pulp sample, collected from a commercial southernUS mill was analyzed and found to have a kappa of 15.5. This unbleachedpulp sample was first digested with a water solution containing sulfurdioxide (SO₂), at 11% consistency, for a period of 2 hours at 90° C.After washing with fresh water and washed water removed, the SO₂pretreated pulp was bleached with bleaching chemicals such as chlorinedioxide (ClO₂), caustic (NaOH) and hydrogen peroxide (H₂O₂) and in athree stage DEpD bleaching sequence. The pulp bleaching in the first Dstage was carried out at 5% pulp consistency, for 50 minutes at 65° C.After pulp washing, washed D pulp was bleached in Ep stage at 11%consistency, for 50 minutes at 85° C. Washed Ep pulp was bleached in thefinal D stage at 11% consistency, for 3 hours at 70° C. The finalbleached pulp was measured for brightness. Using a ClO₂ dosage of 14lbs/bdt in the first D stage, 10 #/bdt of hydrogen peroxide (H₂O₂) and24 #/bdt of NaOH in Ep stage and 8 #/bdt of ClO2 and 3 #/bdt of NaOH infinal D stage, the final bleached pulp has a brightness of 88 ISO. Thetotal ClO2 requirement was 22 #/bdt and total NaOH requirement was 27#/bdt. The pH in the D* stage was from about 2.5 to about 3.5.

Comparative Example 1

Without going through the SO2 digestion stage, the unbleached pulpsample used in Example 1 was bleached with ClO2, NaOH and H2O2 in athree stage DEpD bleaching sequence. First D stage, Ep stage and final Dstage were operated at consistency, time and temperature identical tothose used for D, Ep,D stage of Example I. Using a ClO2 dosage of 20#/bdt in first D stage, 20 #/bdt of H2O2 and 26 #/bdt of NaOH in Epstage and 18 #/bdt of ClO2 and 10 #/bdt of NaOH in final D stage, thefinal bleached pulp has a brightness of 88 ISO. The total ClO2requirement was 38 #/bdt and total NaOH usage was 36 #/bdt.

Comparing results of Example 1 with results of Comparative Example 1showed that with SO2 pretreatment, DEpD bleaching of southern hardwoodto identical 88 ISO brightness can be carried out at much reducedchemical requirement namely total ClO₂ requirement reduced from 38 #/bdtto 22 #/bdt , total NaOH requirement reduced from 36 #/bdt to 27 #/bdtand total H2O2 requirement reduced from 20 #/bdt to 10 #/bdt.

EXAMPLE 2

An unbleached softwood pulp of pine wood species collected from asouthern US mill was analyzed and found to have a kappa of 28.5. Thisunbleached pulp sample was first digested with a water solutioncontaining sulfur dioxide (SO2), at 11% consistency, for a period of 2hours at 90° C. After washing with fresh water and wash water removed,the SO2 pretreated pulp was bleached with bleaching chemicals such aschlorine dioxide (ClO2), caustic (NaOH) and hydrogen peroxide (H2O2) andin a three stage DEpD bleaching sequence. The pulp bleaching in thefirst D stage was carried out at 5% pulp consistency, for 50 minutes at65° C. After pulp washing, washed D pulp was bleached in Ep stage at 11%consistency, for 60 minutes at 80° C. Washed Ep pulp was bleached in thefinal D stage at 11% consistency, for 2 hours at 75° C. After washingthe final D stage pulp was analyzed for brightness. Using a ClO2 dosageof 40 lbs/bdt in the first D stage, 10 #/bdt of hydrogen peroxide (H2O2)and 50 #/bdt of NaOH in Pp stage and 14 #/bdt of ClO2 and 3 #/bdt ofNaOH in final D stage, the final bleached pulp has a brightness of 90ISO. The combined ClO2 usage for two D stage was 54 #/bdt and total NaOHusage for all stages was 53 #/bdt. The pH in the D* stage was from about2.5 to about 3.5.

EXAMPLE 3

An unbleached softwood pulp of pine wood species collected from asouthern US mill was analyzed and found to have a kappa of 28.5. Thisunbleached pulp sample was first digested with a water solutioncontaining sulfur dioxide (SO2), at 11% consistency, for a period of 2hours at 90° C. After washing with fresh water and wash water removed,the SO2 pretreated pulp was bleached with bleaching chemicals such aschlorine dioxide (ClO2), caustic (NaOH) and hydrogen peroxide (H2O2) andin a five stage DEpDED bleaching sequence. The pulp bleaching in thefirst D stage was carried out at 5% pulp consistency, for 50 minutes at65° C. After pulp washing, washed D pulp was bleached in Ep stage at 11%consistency, for 60 minutes at 80° C. Washed Ep pulp was bleached insecond D stage at 11% consistency, for 2 hours at 75° C. Washed second Dstage pulp was bleached in E stage at 11% consistency, for 45 minutes at80° C. Washed E pulp was bleached in final D stage at 11% consistency,for 2 hours at 70° C. After washing, the final D stage pulp was analyzedfor brightness. Using a ClO2 dosage of 28 lbs/bdt in first D stage, 10#/bdt of hydrogen peroxide (H2O2) and 36 #/bdt of NaOH in Ep stage, 14#/bdt of ClO2 in second D stage, 14 #/bdt of NaOH in E stage and 4 #/bdtof ClO2 in final D stage, the final bleached pulp has a brightness of 92ISO. The combined ClO2 usage for all three D stages was 46 #/bdt andcombined NaOH usage for all stages was 50 #/bdt. The pH in the D* stagewas from about 2.5 to about 3.5.

COMPARATIVE EXAMPLE 2

Without going through the SO2 digestion stage, the unbleached pulpsample used in Example 2 was bleached with ClO2, NaOH and H2O2 in a fivestage DEpDED bleaching sequence. Consistency, time and temperature forindividual D, Ep, D, E, D stages were identical those of five DEpDEPstages of Example 3. Using a ClO2 dosage of 56 #/bdt in first D stage,10 #/bdt of H2O2 and 60 #/bdt of NaOH in Ep stage, 14 #/bdt of ClO2 insecond D stage, 14 #/bdt of NaOH in E stage and 4 #/bdt of ClO2 and 1#/bdt of NaOH in final D stage, the final bleached pulp has a brightnessof 90 ISO. The combined ClO2 usage for all three D stages was 74 #/bdtand combined NaOH usage for all stages was 75 #/bdt.

Comparing results of Example 3 with results of Comparative Example 2showed that when bleached with identical five stages DEpDED bleachingsequence, the pretreatment of unbleached southern pine with SO2 stageresulted in three significant process improvement namely, reducing thetotal ClO2 usage from 74 #/bdt to 46 #/bdt, reducing total NaOH usagefrom 75 #/bdt to 50 #/bdt and raising the final pulp brightness from 90ISO to 92 ISO.

Comparing results of Example 2 with results of Comparative Example 2showed that to bleach pine softwood to same 90 ISO brightness, the SO2pretreatment can be used to reduce the requirement of number ofbleaching stages, from five stages DEpDED bleaching to three stages DEpDbleaching. In addition to less major process equipment and lesselectrical power and less steam requirement, it also reduced the totalClO2 requirement from 74 #/bdt to 54 #/bdt.

Various modifications and variations may be devised given theabove-described embodiments of the invention. It is intended that allembodiments and modifications and variations thereof be included withinthe scope of the invention as it is defined in the following claims.

What is claimed is:
 1. A method of making pulp comprising: sulfonatingan alkali pulp at a pH less than 7 prior to, during, or after at leastone bleaching sequence.
 2. The method of claim 1 wherein the sulfonatingof the alkali pulp followed by a washing stage.
 3. The method of claim 1wherein the sulfonating of the alkali pulp is performed during ableaching sequence.
 4. The method of claim 3 wherein the bleachingsequence is prior or during extended delignification stage.
 5. Themethod of claim 3 wherein the bleaching sequence comprises an oxygendelignification stage.
 6. The method of claim 3 wherein the bleachingsequence comprises an ozone delignification stage.
 7. The method ofclaim 3 wherein the bleaching sequence comprises a chlorine-basedbleaching agents.
 8. The method of claim 1 wherein the alkali pulpingprocess is a Kraft pulping process.
 9. The method of claim 1 comprisinga bleaching sequence selected from the group of the formula: D*D₀, D₀D*,D*D₀Eop, D*D₀Ep, OD*(ZD)EpD₁, OD*D₀EpD₁.
 10. The method of claim 1wherein the sulfonating of the alkali pulp is performed by a sulfonatingagent
 11. The method of claim 10 wherein the sulfonating of the alkalipulp selected from the group consisting of sulfur dioxide (SO₂) orsulfur trioxide (SO₃) or sulfurous acid (H₂SO₃), sodium metabisulfite(Na₂S₂O₅), bi-sulfite (MHSO₃) or sulfite (M₂SO₃) where M can be Na, Ca,H, K, Mg or any combination of two or more thereof.
 12. The method ofclaim 1 wherein the pulp pH is from about 2.0 to about 4.0.
 13. Themethod of claim 1 wherein the pulp consistency in the D* stage is fromabout 8% to about 14%.
 14. The method of claim 1 wherein the retentiontime is from about 60 min. to about 120 min.
 15. The method of claim 1wherein the temperature is from about 70° C. to about 120° C.
 16. Themethod of claim 1 wherein the pulp washing stage after the sulfonationstage removes substantially the entire residual sulfonating agent. 17.The method of claim 1 wherein 35% less bleaching agent is used in thebleaching sequence as compared to the same or substantially the samebleaching sequences not sulfonating the alkali pulp to obtain the sameor substantially the same level of pulp brightness after bleaching. 18.The method of claim 1 wherein pulp brightness after bleaching sequencesis at least 3 ISO points greater than the brightness of a pulp by thesame or substantially the same bleaching sequences not sulfonating thepulp.
 19. A sulfonated and fully bleached pulp comprising a compositionof cellulose, hemicelluloses wherein the amount of cellulose containedin the pulp is from about 55% to about 75% by dry weight of the pulp andthe amount hemicellulose contained in the pulp is from about 25% toabout 45% by dry weight of the pulp.
 20. A paper or paperboardcomprising a sulfonated and bleached pulp wherein the amount ofcellulose contained in the pulp is from about 55% to about 75% by dryweight of the pulp and the amount hemicellulose contained in the pulp isfrom about 25% to about 45% by dry weight of the pulp.